The present invention relates to an image forming apparatus wherein an electrostatic image, for example, is developed with a developing agent, and more particularly, to an image forming apparatus wherein an occurrence of image defects caused by a discharge phenomenon of foreign matters in the course of developing can be prevented.
For example, in a certain image forming apparatus for developing an electrostatic image with a developing agent, there are provided an image carrier (hereinafter referred to sometimes as a photoreceptor drum), a developing agent carrier that is close to the image carrier and carries a developing agent, and a clearance regulating member that regulates a clearance between the image carrier and the developing agent carrier, there is used a two-component developing agent, for example, as a developing agent, and a developing agent is supplied from the developing agent carrier to the image carrier to superpose toner images each being different in color for developing while the image carrier impressed with AC bias voltage makes plural turns.
Incidentally, there is a tendency that a thin layer of a two-component developing agent, for example, is supplied on the developing agent carrier so that the clearance is narrowed, and AC bias voltage of high electric field is impressed for the purpose of high speed developing for high quality images.
On the other hand, due to the aforesaid tendency, there is a tendency that image defects which are the so-called ring marks tend to occur. The cause for this is considered that when conductive powder (0.1-0.4 mm in size) passes through a narrow clearance (0.4-0.8 mm) for supplying a thin layer of two-component developing agent to a developing agent carrier, ionization is caused between a image carrier and a tip of the powder to destroy latent images and thereby ring-shaped or disk-shaped (spot-like) latent image defects are formed. It has been confirmed further that the image defects tend to occur or they lead to serious defects when a value of AC voltage is high, the clearance is narrow and a particle of powder is large.
In such an image forming apparatus, there is a tendency that foreign matters sometimes enter a developing unit when replacing or transporting a developing unit, and ring marks are caused suddenly. In that case, image forming operation advances normally for a certain period of time during which ring mark problems decrease, and then, suddenly, the problems happen.